The present invention relates to a dispersion shifted optical fiber and, more specifically, relates to a dispersion shifted optical fiber having a core, an inner clad, and a silica clad which is suitable for the use in wavelength division multiplexing system. In addition, the present invention relates to the dispersion shifted optical fiber which has a low dispersion at a wavelength of 1550 nm, and an effective area of 60 xcexcm2 or more without degradation in bending performance.
The introduction of an optical amplifier in the field of optical communication system makes it possible for a wavelength division multiplexing method to be available. The wavelength division multiplexing method contributes to the increased transmission capacity but causes undesirable non-linear effects that have serious influence on transmission characteristics of the optical fiber. The non-linear effects results from a high power density, i.e., a high power per unit area. Thus, it is possible to reduce the non-linear effects by increasing an effective area of the optical fiber. The increase of the effective area, however, results in the increase of the undesirable bending loss. As such, a new design of the optical fiber has been requested which properly controls both optical characteristics, i.e., the effective area and the bending loss.
Among non-linear effect, four wave mixing creates light having a new wavelength from the interference between lights having different wavelengths, thereby causing the distortion of light signal. The four wave mixing occurs at wavelength where the dispersion is zero, i.e., when lights are in-phase with each other. Thus, if the optical fiber has the low dispersion within erbium-doped fiber amplifier (xe2x80x9cEDFAxe2x80x9d) gain band, the four wave mixing does not occur.
The conventional design of the optical fiber is that a clad having a relatively low refractive index surrounds a core having a relatively high refractive index. Each portion has a larger refractive index than that of pure silica clad by doping a germania thereto. Such design of the optical fiber is disclosed in U.S. Pat. Nos. 5,559,921 and 5,327,516.
The above-mentioned design of the optical fiber makes it possible to obtain the relatively low dispersion at the wavelength of 1550 nm by adjusting radius and the refractive index of the core. Due to the dispersion, the resultant optical fiber can suppress the four wave mixing. However, the optical fiber having this design has a problem caused from the large refractive index difference between the core and the clad: the light concentrates on the core because of the large index difference, and, therefore, the optical fiber can accomplish the decreased bending loss but has the decreased effective area.
Alternatively, there has been a conventional optical fiber design that includes various portions having different radii and refractive indices one another. For example, if the optical fiber design includes three portions, at least one of three portions has a refractive index smaller than or equal to that of pure silica clad and other two portions have refractive indices equal to or larger than that of pure silica clad. Germania is doped into the portions having large refractive indices, whereas Fluoride is doped into the portion with small refractive index. Such design is disclosed in U.S. Pat. Nos. 5,748,824 and 4,852,968.
Such optical fiber design makes it possible for the resultant optical fiber to obtain the low dispersion at the wavelength of 1550 nm by adjusting refractive indices and radii of the various portions in tile optical fiber. As a result, the optical fiber does not suffer from the four wave mixing. The above-mentioned optical fiber structure is designed that the refractive index of each portion is repeatedly increased or decreased in reference to the refractive index of the silica clad. Such distribution of the refractive index results in increasing the effective area and, at the same time, causes the bending loss to increase because the light spreads through various portions. Thus, the above-mentioned optical fiber can provide the large effective area but still has a problem of the increased bending loss.
In order to solve the above-mentioned problems in the prior art, the present invention provides an optical fiber having triple clad, which is suitable for the use in the wavelength division multiplexing system that is served by erbium-doped fiber amplifier.
It is an object of the present invention to provide the optical fiber that has a relatively low dispersion at the wavelength of 1550 nm to suppress the four wave mixing.
In addition, it is the object of the present invention to provide the optical fiber that has an effective area equal to or larger than 60 xcexcm2 along with the decreased bending loss.
In order to accomplish the above-mentioned object, the present invention provides a dispersion shifted optical fiber, which comprises: a core having a refractive index distribution (ncore); an inner clad that surrounds the core and has a refractive index distribution (nclad); and, a silica clad surrounding the inner clad and having a refractive index (nsilica). The core comprises: a first core having a refractive index (n1) that is uniform over its radius (a1); and a second core surrounding the first core and having a refractive index (n2) that is gradually decreased over its radius (a2). The inner clad comprises: a first inner clad surrounding the second core and having a refractive index (n3) that is uniform over its radius (a3) and a second inner clad surrounding the first inner clad and having a refractive index (n4) that is uniform over its radius (a4); and, wherein the refractive indices of the first and second cores, the first and second clads, and the silica clad satisfy a relationship of n1 greater than n2 greater than n3 greater than n4 greater than nsilica.
According to other aspect, the present invention provides a dispersion shifted optical fiber, which comprises: a core having a refractive index distribution (ncore); an inner clad that surrounds the core and has a refractive index distribution (nclad); and, a silica clad surrounding the inner clad and having a refractive index (nsilica). The core comprises: a first core having a refractive index (n1) that is gradually increased from an initial refractive index (n0) over its radius (a1); and a second core surrounding the first core and having a refractive index (n2) that is gradually decreased over its radius (a2). The inner clad comprises: a first inner clad surrounding the second core and having a refractive index (n3) that is uniform over its radius (a3) and a second inner clad surrounding the first inner clad and having a refractive index (n4) that is uniform over its radius (a4). The maximum refractive index of the second core is larger than a minimum refractive index of the first core, and the refractive indices of the second core, the first and second inner clads and the silica clad satisfy a relationship of n2 greater than n3 greater than n4 greater than nsilica.
According to another aspect, the present invention provides optical fiber that accomplishes both large effective area and decreased bending loss by adjusting radii and refractive indices of two inner clads.
Based on the above structure, the present invention can provide the dispersion-shifted optical fiber having triple clad which is suitable for the use in the wavelength division multiplexing communication system; the optical fiber according to the present invention can suppress the non-linear effects by having the low dispersion and the effective area of 60 xcexcm2 or more at the wavelength of 1550 nm. In addition, the optical fiber according to the present invention can accomplish increased effective area and decreased bending loss at the same time.
In addition, the present invention provides the optical fiber that can be easily fabricated through simplified fabrication process because of the simple refractive index profile.